In known fuel injector designs, a nozzle control valve is provided to control movement of a fuel injector valve needle relative to a seating and, thus, the delivery of fuel from the injector. A so-called Electronic Unit Injector (EUI) is an example of such an injector. An EUI includes a dedicated pump having a cam-driven plunger for raising fuel pressure within a pump chamber, and an injection nozzle through which fuel is injected into an associated engine cylinder. A metering valve is operable to control the pressure of the fuel within the pump chamber. When the metering valve is in an open position, the pump chamber communicates with a low pressure fuel reservoir so that fuel pressure within the pump chamber is not substantially affected by movement of the plunger and fuel is simply drawn into and displaced from the pump chamber as the plunger reciprocates. Closure of the metering valve causes pressure in the pump chamber to rise as the plunger is driven to reduce the volume of the pump chamber. Each EUI also has an electronically controlled nozzle control valve that is arranged to control the timing of commencement and termination of the injection of fuel into an associated engine cylinder. Typically, the engine is provided with a plurality of EUIs, one for each cylinder of the engine.
Although the use of a nozzle control valve in an EUI provides a capability for controlling the injection timing, and such units are capable of achieving high injection pressures, both injection pressure and injection timing are limited to some extent by the nature of the associated cam drive.
In common rail fuel injection systems, a single pump is arranged to charge an accumulator volume, or common rail, with high pressure fuel for supply to a plurality of injectors of the fuel system. As in an EUI, the timing of injection is controlled by means of a nozzle control valve associated with each injector. One advantage of the common rail system is that the timing of injection of fuel at high pressure is not dependent upon a cam drive, and so the flexibility of injection timing is good. However, achieving very high injection pressure within a common rail system is problematic and requires a dedicated high pressure pump of significant cost.
Recognising that both EUI and common rail systems have certain disadvantages, in granted U.S. Pat. No. 7,047,941 (Delphi Technologies Inc.) the Applicants have previously proposed a hybrid fuel injection system which combines the functionality and benefits of both types of system, whilst avoiding several of the drawbacks of each of them.
FIG. 1 shows a hybrid system of the aforementioned type including a common rail fuel pump 10 which supplies fuel at a moderately high and injectable pressure level (e.g. 300 bar) to a common rail 12. This is referred to as the first pressure level. The common rail 12 supplies pressurised fuel to a first supply passage 14 which communicates with a pump chamber 16 under the control of a rail control valve 18. The pump chamber 16 forms part of a pump arrangement including a pumping plunger 20 that is driven by means of a driven cam 22, typically a roller and rocker mechanism. The pump chamber 16 supplies fuel to a dedicated fuel injector 24 which is separated from the pump chamber 16 by a second supply passage 26. The fuel injector 24 is arranged to inject fuel into the engine when a valve needle 28 of the injector is caused to lift under the control of an injector control valve 30.
The fuel injection system of FIG. 1 has two key modes of operation. If the rail control valve 18 is closed, movement of the plunger 20 under the influence of the cam 22 causes fuel within the pump chamber 16, which is initially at a relatively low level, to be increased to a variable, higher pressure level. Fuel at this second, variable higher pressure level is then delivered through the second supply passage 26 to the injector 24 and is delivered to the engine under the control of the injector control valve 30. If, however, the rail control valve 18 is open, the action of the plunger 20 has no pressurizing effect on fuel within the pump chamber 16 and so fuel is delivered to the injector 24 at the first pressure level. By controlling the status of the rail control valve 18 it is therefore possible to vary the injection pressure between the first and second pressure levels.
Whilst the hybrid system in FIG. 1 provides many advantages over the more conventional systems, it is not compatible with many existing engine installations. Where manufacturers have invested heavily in production line facilities for one type of engine installation, the cost of re-tooling can be prohibitive to manufacturing different types of engine.
It is with a view to addressing this problem in particular that the invention provides an improved fuel injection system which is compatible with many existing assembly line facilities.